1. Field of the Invention
The present invention pertains to digital image processing. More particularly, this invention relates to automatically reducing red-eye in a digital image while maintaining the natural appearance of the eyes.
2. Description of the Related Art
As is known, photographing a person in a relatively dark environment requires flashlight to avoid under-exposure. The use of flashlight, however, often results in a person's eyes being red in the photograph, making the person look "spooky" or unnatural in the photograph. This is typically referred to as the "red-eye" phenomenon or simply red-eye.
The red-eye typically results from the person's pupils not being able to quickly adjust to the flashlight in darkness. As is known, the pupils of a person are enlarged in a dark environment. When flashlight appears, the pupils are not able to reduce their sizes due to the suddenness of the flashlight. This typically causes the flashlight reflecting off the retina at the back of the eyes, causing red-eye.
Several prior art techniques have been proposed to reduce the red-eye effect. A common prior art approach is to use multiple flashes in the camera to contract the pupils before a final flash is used to expose and capture the image. However, disadvantages are associated with this prior art approach. One disadvantage is the delay between the time when the first flashlight appears and the time when the picture is actually taken. This means the picture is taken several seconds after the exposure button has been pressed. This may cause confusion and the subjects may move away from the posed positions before the image is captured. Moreover, the red-eye problem still occurs when the user forgets to enable this feature of the camera during photographing, or when the camera is not equipped with such red-eye prevention feature. Further, this prior art approach cannot solve the red-eye problem in the already-taken photos.
With the advance of image processing technologies, it is now possible to digitize an image and store the digitized image in a computer system. This is typically done either using a digital camera to capture the image digitally, or using a scanner that converts the image into digital form. The digital image includes data representing image pixels arranged in a matrix. The data of the digital image are then stored in the computer. The digital image can be retrieved for display and can also be digitally altered in the computer.
Because images can now be captured as or converted into digital images, it is thus possible to correct the red-eye problem in an image digitally. Some prior art schemes have been proposed to correct the red-eye problem digitally. One such prior art scheme simply provides the user with means for manually painting over the red eyes digitally. The disadvantage of this prior art scheme is that some kind of painting skill is needed for the user to paint over the red eyes. Another disadvantage is that the correction of the red-eye is not done automatically, but rather manually.
Another prior art approach requires the user to precisely locate the center of a pupil so that a black circle is placed over the red-eye region. The disadvantage of this prior art approach is that the red-eye region is often not a circular region. This may cause portions of the red-eye region not to be covered by the black circle. In addition, the black circle may not be able to cover the peripheral area (i.e., the pink ring) of the red-eye region. Moreover, replacing the red pupil with a complete black circle may also cover the glint in the pupil. As is known, the glint in the pupil is usually a bright "white" spot. Thus, the result of this type of correction is often quite noticeable and undesirable, and sometimes destroys the natural appearance of the eyes in the image.